1. Field of the Invention
The present invention relates to an antiparallel (AP) pinned spin valve sensor with a pinning layer reset circuit and electrostatic discharge (ESD) protection circuit and, more particularly, to an ESD protection circuit for an AP pinned spin valve sensor that is compatible with a reset circuit so that objectives of both circuits can be fully realized.
2. Description of the Related Art
A spin valve sensor is employed by a read head for sensing magnetic fields from moving magnetic media, such as a magnetic disk or a magnetic tape. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer, and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned 90.degree. to the magnetization of the free layer and the magnetization of the free layer is free to respond to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal; when the magnetizations of the pinned and free layers are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to sin .theta., where .theta. is the angle between the magnetizations of the pinned and free layers. A spin valve sensor is characterized by a magnetoresistive (MR) coefficient that is substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. For this reason a spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor.
A read head employing a spin valve sensor (hereinafter referred to as a "spin valve read head") may be combined with an inductive write head to form a combined magnetic head. In a magnetic disk drive, an air bearing surface (ABS) of a combined magnetic head is supported adjacent a rotating disk to write information on or read information from the disk. Information is written to the rotating disk by magnetic fields which fringe across a gap between the first and second pole pieces of the write head. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
An improved spin valve sensor, which is referred to hereinafter as an antiparallel (AP) pinned spin valve sensor, is described in commonly assigned U.S. Pat. No. 5,465,185 to Heim and Parkin which is incorporated by reference herein. The AP pinned spin valve sensor differs from a single film pinned layer spin valve sensor, described above, in that the pinned layer of the AP pinned spin valve sensor comprises multiple thin films, which arc collectively referred to as an antiparallel (AP) pinned layer. The AP pinned layer has a nonmagnetic spacer film sandwiched between first and second ferromagnetic thin films. The first thin film is exchange coupled to the antiferromagnetic layer by being immediately adjacent thereto and has its magnetic moment directed in a first direction. The second thin film is immediately adjacent to the free layer and is exchange coupled to the first thin film by the minimal thickness (in the order of 5 .ANG.) of the spacer film between the first and second thin films. The magnetic moment of the second thin film is oriented in a second direction that is antiparallel to the direction of the magnetic moment of the first film. The magnetic moments of the first and second films subtractively combine to provide a net pinning moment of the pinned layer. The direction of the net moment is determined by the thicker of the first and second thin films. The thicknesses of the first and second thin films are chosen to reduce the net moment. A reduced net moment equates to a reduced demagnetization (demag) field from the AP pinned layer. Since the antiferromagnetic exchange coupling is inversely proportional to the net pinning moment, this increases exchange coupling.
A high exchange coupling promotes higher thermal stability of the head. When the head encounters elevated thermal conditions caused by electrostatic discharge (ESD) from an object or person, or by contacting an asperity on a magnetic disk, the blocking temperature of the antiferromagnetic layer can be exceeded, resulting in disorientation of its magnetic spins. The magnetic moment of the pinned layer is then no longer pinned in the desired direction.
Significant advantages of the AP pinned spin valve over the typical single film pinned layer include a greater exchange coupling field and a lower demag field, which enhance thermal stability in the AP pinned spin valve sensor.
ESD is particularly troublesome for both the single pinned layer spin valve sensor and the AP pinned spin valve sensor. Multiple magnetic heads employing the sensors are made in rows and columns on a wafer. After construction of the heads on the wafer, the wafer is diced into rows. At the wafer and/or the row level the antiferromagnetic pinning layer of either spin valve sensor is subjected to heat above its blocking temperature in the presence of a magnetic field to orient the magnetic spins of the antiferromagnetic pinning layer in a direction that is perpendicular to the ABS. After dicing the row into individual magnetic heads, each head is manually mounted by an assembler on a suspension of a magnetic disk drive. When the magnetic head is mounted on the suspension it is typically connected to an ESD protection circuit. Unfortunately, before the head is mounted on the suspension it is at risk of ESD from the assembler or contact with another object that may destabilize the magnetic spins of the antiferromagnetic pinning layer. A multiple head disk drive can be rendered unmarketable if only one head is destabilized. Accordingly, there needs to be a reset circuit at the suspension level or on the magnetic disk drive for resetting the magnetic spins of the antiferromagnetic pinning layer. The problem is providing reset and ESD circuits that are compatible with one another.
It is necessary that the ESD circuit provide adequate ESD protection while allowing the reset circuit to apply a voltage with a sufficiently high pulse to reset the antiferromagnetic pinning layer. It is also necessary that the ESD circuit provide sufficient turn-on voltage in the direction of the sense current to prevent shunting of the sense current. Diodes (which are used for ESD protection) inherently have a small amount of leakage current which may be decreased by placing multiple diodes in the direction of the sense current. Shunting of the sense current through ESD protection diodes significantly impacts the readback signal. Accordingly, sufficient turn-on voltage must be employed in the direction of the sense current to substantially eliminate sense current leakage. Another factor bearing on compatibility is that in both the single film pinned layer spin valve sensor and the AP pinned spin valve sensor the sense current must be in a predetermined direction to balance a sense current field with other fields and influences on the spin valve sensor which is discussed next.
Both the single film pinned layer spin valve sensor and the AP pinned spin valve sensor demonstrate an AMR influence on the free layer that is also characteristic of an AMR sensor. This is because of the relative rotation between the directions of the magnetic moment of the free layer of the spin valve sensor and the sense current. AMR is employed in the AMR sensor for detecting signals and is due to a change in resistance of an MR stripe as the magnetic moment of the MR stripe rotates relative to the sense current in response to magnetic fields from a rotating disk. The AMR may substantially change the position of the bias point of the spin valve sensor relative to positive and negative readback signals detected by the sensor. The influence of the AMR effect on the free layer must therefore be dealt with in establishing the bias point.
The transfer curve (a plot of the readback signal of the spin valve head versus the applied signal from the magnetic disk) for a spin valve sensor is defined by sin .theta., where .theta. is the angle between the directions of the magnetic moments of the free and pinned layers. A substantially flat portion of the transfer curve is selected for location of the bias point so that response of the sensor is substantially linear. Since positive and negative magnetic fields from a moving magnetic disk are typically equal in magnitude, it is important that positive and negative changes in the GMR of the spin valve sensor about the bias point on the transfer curve also be equal.
The location of the bias point on the transfer curve is influenced by four major forces on the free layer: a ferromagnetic coupling field H.sub.FC between the pinned layer and the free layer; a demag field H.sub.demag on the free layer from the pinned layer; a sense current field H.sub.SC from all conductive layers of the spin valve (except the free layer; and, the influence of the AMR. The influence of the AMR on the bias point is the same as a magnetic influence thereon and it can be defined in terms of magnitude and direction. This influence is referred to herein as the AMR EFFECT.
In the single pinned layer spin valve, H.sub.demag and the AMR effect are balanced by H.sub.FC and H.sub.SC. In the AP pinned spin valve sensor the AMR effect is balanced by H.sub.demag, H.sub.FC and H.sub.SC for the purpose of promoting symmetry of the read signal.